The present invention relates to a silicon nitride cutting tool with improved properties for metal cutting applications in cast iron.
Silicon nitride has been recognized as a cutting tool material for machining of cast iron due to its good wear resistance and good high temperature properties. During the last decade, the development of the material has led to an increased use in metal cutting applications and with more advanced silicon nitride material with improved properties, the potential will grow.
In U.S. Pat. No. 5,382,273, a silicon nitride based ceramic material for a cutting tool is described. The material is a beta silicon nitride with less than 5 weight % of an intergranular phase consisting of magnesia and yttria. This material is disclosed to have improved metal cutting performance, improved hardness at 1000.degree. C., good transverse rupture strength and improved Weibull modulus compared to prior art.
Pyzik et al. have described in a number of patents a self-reinforced silicon nitride that exhibits high fracture toughness and high fracture strength. In U.S. Pat. No. 5,312,785, a process of producing a self-reinforced silicon nitride comprising a glassy phase, a second crystalline phase of zirconium oxide and a crystalline phase of a metal zirconium silicide and/or metal zirconium silicon nitride is described. In U.S. Pat. No. 5,160,508, a self-reinforced silicon nitride ceramic of high fracture toughness is described. The material contains a i) crystalline beta silicon nitride phase with at least 20 volume % of the beta silicon nitride in the form of whiskers having an aspect ratio greater than 2.5, a glassy second phase containing ii) densification aid; iii) conversion aid; iv) an aid which enhances the growth of beta silicon nitride whisker; and v) silica. U.S. Pat. No. 5,120,328 describes a method of manufacturing dense self-reinforced silicon nitride by pressureless or low pressure gas sintering. The silicon nitride body comprises at least 20 volume % beta silicon nitride whiskers, 2-10 weight % of a glassy phase, 0.5-5 weight % of a second crystalline phase of zirconium oxide and optionally 0.1-3 weight % of a crystalline phase of a metal zirconium silicide and/or metal zirconium silicon nitride. In U.S. Pat. No. 5,118,645, a process of preparing a silicon nitride body from a powder mixture of silicon nitride and densification, conversion, whisker enhancing and Palmqvist toughness enhancing aids is described. U.S. Pat. No. 5,091,347 describes a process for preparing a silicon nitride from a mixture of i) silicon nitride and ii) silicon dioxide as densification aid; iii) a conversion aid; and iv) a whisker growth enhancing aid at sintering temperatures above 1750.degree. C. and pressures of at least 20.7 MPa.
In U.S. Pat. No. 4,497,228, an abrasion resistant silicon nitride is described. This is achieved by adding up to 60 volume % of hard particles of refractory metal carbides and nitrides or combinations thereof.